Help me with this PHP error - Cannot modify header

Warning : Cannot modify header information - headers already sent by (output started at /home/aauaforum/public_html/login.php:1) in /home/aauaforum/public_html/login.php on line23Warning : Cannot modify header information - headers already sent by (output started at /home/aauaforum/public_html/login.php:1) in /home/aauaforum/public_html/login.php on line